The present invention relates to a hydraulic control apparatus for controlling electric current for exciting a solenoid of a solenoid controlled valve.
A known hydraulic control apparatus of this kind is shown in FIG. 16 which is a block diagram of the apparatus. In this hydraulic control apparatus, an npn transistor 3 is interposed between a solenoid 1 of a solenoid controlled valve and a DC power supply 2 in such a manner that the emitter E of the transistor 3 is connected to the solenoid 1 and the collector C thereof is connected to the DC power supply. A pulse-width modulated control signal Vc, whose ON/OFF time ratio that is a ratio of ON time to OFF time varies, is applied to the base B of the transistor 3. Only when the control signal Vc is ON, electric current flows between the collector C and the emitter E, so that the solenoid 1 is excited.
The control signal Vc is generated as follows: Electric current I which flows through the solenoid 1 is fed back to a subtractor 6 as a voltage signal Vf through a noise removing low-pass filter 5. The subtractor 6 determines a deviation between the voltage signal Vf and a command signal Vo indicative of a target stop position of a spool (not shown). A deviation signal Ve indicative of the deviation is inputted to the noninverting terminal of a comparator 8 through an amplifier 7. To the inverting terminal of the comparator 8 is applied an AC signal from an oscillator. The AC signal Vd has a dither frequency of 100-300 Hz applied to prevent the spool of the solenoid controlled valve from being hydraulically locked to a cylindrical chamber. The comparator 8 determines a difference between the levels of the signals Ve and Vd inputted to both terminals thereof, thus outputting a pulse signal which becomes ON when the difference between Ve and Vd (Ve-Vd) is positive. Accordingly, the higher the level of the deviation signal Ve, with respect to the level of the AC signal Vd, outputted from the amplifier 7 becomes in a direction shown by an arrow U in FIG. 16, the greater the ON/OFF time ratio of the pulse signal whose ON time is shown by heavy lines is, whereas as the level of the deviation signal Ve becomes lower, the ON/OFF time ratio of the pulse signal decreases. Thus, the deviation signal Ve is pulse-width modulated to produce the control signal Vc which is applied to the base B of the transistor 3. The higher the level of the deviation signal Ve is, the stronger the solenoid 1 is excited. As a result, the spool is displaced to a great extent and a negative feedback of the spool displacement to the subtractor 6 is performed. The spool is displaced until the level of the deviation signal Ve becomes 0, when the spool is at the target stop position. A diode 4 disposed in parallel with the solenoid 1 serves as a means for reducing a surge voltage which is generated when the transistor 3 is deenergized.
According to the hydraulic control apparatus having the above-described construction, the exciting current of the solenoid controlled valve is controlled by feeding back the signal to the transistor 3 through the subtractor 6, the amplifier 7 and the comparator 8, and irrespective of the strength of the exciting current, a dither in the range from 100-300 Hz is applied to the spool so that the spool can be prevented from being hydraulically locked to the cylindrical chamber and the transistor 3 is prevented from heating and the temperature thereof is prevented from rising even though the transistor 3 is kept to be ON.
However, in the above-described hydraulic control apparatus, the AC signal Vd having the same frequency (100-300 Hz) as that of a dither is employed as a carrier wave to pulse width-modulate the deviation signal Ve. Accordingly, the deviation signal Ve which changes in a frequency higher than the dither frequency Vd cannot be pulse width-modulated. Therefore, when the command signal having a high frequency is inputted to the noninverting terminal of the subtractor 6, as shown in FIG. 17a which is a Bode plot representing the characteristics of gain versus frequency, the gain of electric current supplied to the solenoid 1 through the transistor 3 is rapidly reduced at a frequency of more than 20 Hz and fluctuates to a great extent in the vicinity of a frequency of 100-300 Hz. Thus, the spool of the solenoid controlled valve cannot be controlled as desired. In addition, since the frequency of the AC signal Vd is as low as 100-300 Hz, it is necessary to use the low-pass filter whose the time constant is great. Therefore, the phase delay of the feedback signal is outstanding. As shown in FIG. 17b which is a Bode plot representing a characteristics of phase shift versus frequency, the difference between the phase of the electric current supplied to the solenoid 1 and the phase of the command signal becomes great at the frequency of more than 20 Hz and changes extremely in the vicinity of the frequency of 100-300 Hz. Thus, the spool cannot be controlled as desired.
FIG. 18 shows the control characteristics of the hydraulic control apparatus shown in FIG. 16 measured when it is applied to a solenoid controlled proportional throttling control valve which controls the supply of hydraulic fluid to and the discharge thereof from the swash plate control cylinder of a variable displacement hydraulic pump. The hydraulic control apparatus is incapable of favorably controlling the spool of the control valve when the frequency of the command signal is in the range from 100 to 300 Hz as described above. Therefore, as shown in FIG. 18, when command signals Vop and Voq indicating the deliveries pressure and the discharge amount of hydraulic fluid are changed stepwise, it takes 0.3 seconds for the exciting current Ic and the swash plate to become steady-state. That is, the response characteristics of electric current and flow rate of the hydraulic fluid are unfavorable. Further, FIG. 18 indicates that it takes about 0.1 seconds for the delivery pressure of the hydraulic fluid to rise to an appropriate degree and that the delivery pressure fluctuates in a great extent after it has risen to the appropriate degree.